The present invention relates to a braking device with fluid control, generally electrohydraulic or electropneumatic, intended for the braking of trailers provided with mechanical brakes, as well as a safety valve usable particularly with this device.
Various control systems for braking of a trailer attached to a tractor vehicle have been known for a long time, particularly the system called inertial, the action of which is controlled by the deceleration of the tractor vehicle, but the use of which is prohibited by certain laws for trailers of which the load exceeds a fixed value. Another known system is that of which the control is pneumatic, necessitating the installation of a compressor on the tractor vehicle and the passage of pneumatic conduits between the tractor vehicle and the trailer; though this system is efficient, it is costly, impractical, and relatively polluting to the extent that the air loaded with residues of oil and of antifreeze is returned to the ambient air. As a variation of the preceding system, the depression system may be encountered, the depression necessary for the control of the brakes of the trailer being drawn from the engine: this system has a relatively long response time and also comprises impractical pneumatic conduits between the tractor vehicle and the trailer. In agriculture, where the tractors are generally equipped with a central hydraulic unit, the trailer braking control can take place hydraulically, a hydraulic conduit connecting the tractor to the trailer. Another known braking system is the electric system, an electric signal being supplied to an electromagnet disposed on the brake flange, causing it to lie flat against a disk driven by the wheel of the trailer, this driving movement actuates an eccentric controlling the distance between the shoes disposed within the brake drum; this system, though it has the advantage of not necessitating any pneumatic or hydraulic conduit between the tractor vehicle and the trailer, but solely an electric conduit clearly more practical than the preceding ones, presents the drawbacks that the regulation of the braking forces between the two or the four wheels of the trailer is rather difficult and that braking effectiveness depends very much upon the temperature of the disk.
The published application DE-A-3,133,102 describes a braking device by which the control between the tractor vehicle and the trailer takes place by electric means. However, this device necessitates rather complex electronic means in order to create a non-continuous electric signal, a function of the desired braking action, which is compared with another electric signal representing the instantaneous hydraulic pressure prevailing within the hydraulic braking circuit. The signal resulting from this comparison generates, by other electronic means, two signals acting differentially on two valves of the hydraulic circuit. Besides the fact that considerable electronic means are necessary in this device, the differential action on two different valves may create undesirable braking effects in case of imbalance between the two control circuits, hence between the two valves.
Another device is described in the patent US-A-3,951,464, where the control of the hydraulic circuit is also obtained by electric means. In this case, the electric braking-control signal acts directly upon a motor controlling a pump delivering the oil pressure necessary for braking; this configuration renders a braking effect proportional to the required action impossible. On the other hand, in order to be able to have a braking action when the trailer is unhooked, hence when the connecting cable is disconnected, an additional battery is necessary on the trailer. This additional element is costly and requires maintenance.